Pickling is the process of chemically removing oxides and scale from the surface of a metal by the use of acids such as sulfuric, nitric, hydrochloric and hydrofluoric. The reaction between the metal being treated and the acid also includes the solution of metal as a salt of the acid and the evolution of hydrogen.
The rate of pickling is affected by the acid concentration, temperature and time of metal immersion. The nature of the metal oxide present, such as FeO, Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4, will also influence the rate of pickling.
Since the acid in the pickling operation is gradually consumed by the removal of the oxides and scale, additional fresh acid is added along with water to maintain a uniform cleaning operation. The acid in the pickling tanks is therefore regularly monitored and maintained within relatively critical levels.
In this regard, various measuring systems for controlling pickling conditions and measuring acid concentrations have been used.
U.S. Pat. No. 3,062,223 relates to sulfuric acid measurements and uses a controller connected to a conductivity probe having temperature compensation. Changes in acid concentration modify the conductivity of the bath. It was found that an increase in ferrous sulfate in the pickle liquor caused a rise in the bath density and a drop in bath temperature also caused an increase in density. The decomposition of the acid which formed ferrous sulfate influenced the conductivity readings and so did the temperature changes. Since the monitors could not determine if the temperature had dropped or if there had been an increase in ferrous sulfate, the temperature was controlled. It was critical to the process to determine if part of the bath needed to be bled off to remove ferrous sulfate or fresh acid of higher concentration needed to be added with the appropriate dilution in water. To understand which changes in bath conditions were affecting bath density and conductivity, the temperature was maintained relatively constant.
U.S. Pat. No. 3,074,277 discusses an automatic control system for sulfuric acid pickling. The system involves the independent measurement of specific gravity and ferrous sulfate concentration and combines the signals to correct the specific gravity for the component contributed by the ferrous sulfate. A net signal is obtained which is a measure of the acid concentration and can be used to control the addition of makeup acid.
U.S. Pat. No. 3,262,051 discloses a system which determines the concentration of nitric acid in a bath by using current density measurements. The voltage across a pair of electrodes disposed within a bath with a selected current density through the bath provides a strong signal for a small change in acid concentration. Slight corrections for salt formation are made in the current density if the bath is originally salt-free.
U.S. Pat. No. 3,427,198 is an improvement to U.S. Pat. No. 3,074,277 and teaches a system that maintains the ferrous sulfate constant so that the specific gravity measurement is directly proportional to the acid concentration. The ferrous sulfate is held constant by water additions using a colorimeter to determine the metal ion concentration. Hydrochloric acid pickling is also described with the ferrous chloride held constant.
U.S. Pat. No. 3,433,670 describes an automatic pickling analysis system for several acids including hydrochloric acid. A colorimeter system measures the color absorption of the ferrous salts in the acid bath. The specific gravity, with compensation for the ferrous salt, is also determined.
Prior acid analyzing systems have also included titration means for determining acid concentrations in pickling operation. However, these means are difficult to standardize and do not provide a system which is simple and easily adapted into an automatic control system.
The patents discussed above have noted that the presence of the iron salt in the pickle liquor complicates the determination of acid concentration. Various analytical approaches have included holding the salt constant, holding the temperature constant and correction for the readings. The present invention has discovered a system which is far easier to control by computer and automatically produces reliable and consistent measurements of the acid concentrations. The present invention is the first to recognize that the influence of metal salt on conductivity measurements may be minimized by using a controlled concentration of acid.